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In this work we describe the Horizontal Ice Nucleation Chamber (HINC) as a new instrument to measure ambient ice-nucleating particle(INP) concentrations for conditions relevant to mixed-phaseclouds. Laboratory verification and validation experiments confirmthe accuracy of the thermodynamic conditions of temperature (T)and relative humidity (RH) in HINC with uncertainties in Tof ±0.4 K and in RH with respect to water(RHw) of ±1.5 %, which translatesinto an uncertainty in RH with respect to ice(RHi) of ±3.0 % at T > 235 K. For further validation of HINC as a fieldinstrument, two measurement campaigns were conducted in winters 2015and 2016 at the High Altitude Research Station Jungfraujoch (JFJ;Switzerland, 3580 m a. s. l. ) to sample ambient INPs. Duringwinters 2015 and 2016 the site encountered free-troposphericconditions 92 and 79 % of the time, respectively. We measuredINP concentrations at 242 K at water-subsaturated conditions(RHw = 94 %), relevant for the formation ofice clouds, and in the water-supersaturated regime(RHw = 104 %) to represent ice formationoccurring under mixed-phase cloud conditions. In winters 2015 and2016 the median INP concentrations at RHw = 94 % was below the minimum detectable concentration. AtRHw = 104 %, INP concentrations were anorder of magnitude higher, with median concentrations in winter 2015of 2.8 per standard liter (std L−1; normalized tostandard T of 273 K and pressure, p, of1013 hPa) and 4.7 std L−1 in winter 2016. Themeasurements are in agreement with previous winter measurementsobtained with the Portable Ice Nucleation Chamber (PINC) of2.2 std L−1 at the same location. During winter 2015,two events caused the INP concentrations at RHw = 104 % to significantly increase above the campaignaverage. First, an increase to 72.1 std L−1 was measuredduring an event influenced by marine air, arriving at the JFJ fromthe North Sea and the Norwegian Sea. The contribution fromanthropogenic or other sources can thereby not be ruled out. Second,INP concentrations up to 146.2 std L−1 were observedduring a Saharan dust event. To our knowledge this is the first timethat a clear enrichment in ambient INP concentration in remoteregions of the atmosphere is observed during a time of marine airmass influence, suggesting the importance of marine particles on icenucleation in the free troposphere

atmospheric chemistry and physics

The Horizontal Ice Nucleation Chamber (HINC): INP measurements at conditions relevant for mixed-phase clouds at the High Altitude Research Station Jungfraujoch